2 Answers
2

Professor Zoom taunting The Flash over the death of the hero's mother is responsible for the timeline change. In the end, it is revealed that The Flash is responsible for the creation of the alternate world because, after Zoom's taunting, he traveled back in time to save his mother. This created a chain of events which led the world to a destructive situation. Professor Zoom took advantage of this situation.

Many things are implied with the film and require some knowledge of the comics. Usually, films set-up the background within themselves; instead, this film makes some very minor assumptions on the part of the audience.

As a time-traveling speedster, Professor Zoom is immune to the changes in the timestream as much as Barry Allen was (for as long as he could hold out; we witness him eventually merge with his alternate universe memory history). At any point, Zoom could have gone back into the past to observe Barry stopping the crime that killed his mother to gain knowledge of how that changed the timeline.

The motivation of Professor Zoom is to torture Barry Allen. He lets Barry know that by Barry having altering history to save one life, Barry caused the chain of events that ends with the world on the brink of destruction. Zoom took no part in this other than to be an observer and to get Barry to find him to taunt him.